Unlike some others in this thread, I don't think it's impossible to use UDP to send data in two directions: a heartbeat from the client to the server, and data from the server to the client. In fact, I don't see any particular difficulties -- with IP over 30 years old, we have learned to distinguish between source and destination addresses in IP packets (not that this ever was a problem).

You want to do something like the following (untested pseudo code):

... create sockets ...
my @fileno = (fileno(SOCKET1),
              fileno(SOCKET2),
              fileno(SOCKET3),
              fileno(SOCKET4));
my ($rbits, $ebits);
vec($rbits, $_, 1) = 1 for @fileno;
vec($ebits, $_, 1) = 1 for @fileno;
while (1) {
    my $time_to_next_heartbeat = ...;  # How long till we have to send
+ a hb
    if ($time_to_next_heartbeat <= 0) {
        ... send heartbeats ...
        redo;
    }
    if (select (my $rbits = $rbits, undef, my $ebits = $ebits, $time_t
+o_next_heartbeat)) {
        if ($ebits) {
            ... deal with errors ...
        }
        if ($rbits) {
            foreach my $fileno (@fileno) {
                if (vec($rbits, $filen0, 1)) {
                    ... read data ...
                }
            }
        }
    }
}
[download]

There's a much simpler solution though, and that may work for you:

• Start 4 processes. Each process sends a heart beat every 10 seconds (one process for each of the ports).
• Start another 4 processes. Each of them reads (and logs) whatever it gets from a particular socket.

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